Radio frequency catheter ablation in the heart is now well known. In this therapy small lesions are formed that interrupt pathways of muscular excitation and terminate certain arrhythmias. There are some specialized applications where larger or longer lesions are necessary to achieve a successful therapy. As an example, there is a need for one or more such lesions to terminate atrial flutter and atrial fibrillation. In these cases, the lesions need to exceed one centimeter in length and be uninterrupted.
One method of producing such a lesion is to drag a tip electrode, preferably with a thermocouple attached along the surface of an atrium while applying RF energy. The thermocouple allows the operator of the catheter to measure the temperature in the tip electrode to ensure good ablation of the heart tissue. However, in the beating heart, the tip electrode may hop or swerve sideways, especially in trabeculated areas, producing intermittent lesions.
Another method of producing lesions longer than one centimeter in length without gaps is by using a long spiral electrode that wraps around the catheter body multiple times. This creates multiple lesions which overlap, thus creating a continuous long lesion. However, a spiral electrode has the disadvantage of producing poor electrocardiograms due to its length. It is also difficult to measure temperature that is representative of the ablation site.
Several practitioners in the field of electrophysiology and especially RF ablation of atrial arrhythmias have suggested the use of multiple, preferably ten, spaced-apart ring electrodes. Such a catheter, however, is difficult to construct for several reasons. For example, soft copper lead wires are typically used to connect the ring electrodes with a high frequency generator to create the radio frequency ablation at the ring electrodes. Soft copper is needed to deliver the RF energy and allow a cost effective catheter design. However, a RF ablation catheter has to be relatively small, about 8 French, for placement inside the heart. The inner diameter of the catheter is even smaller and restricts the number of wires that can be placed in the catheter. Therefore, the copper wire must be large enough to carry at least one ampere of current, yet small enough to fit within the catheter. Very small diameter copper wires, e.g., No. 40 copper wires, would be ideal for use in a 10 electrode ablation catheter, but are too fragile to allow for manipulation and catheter construction. Hence, it is difficult to get enough copper lead wires, e.g., 20 (10 lead wires for the electrodes and 10 lead wires for temperature sensors), to the electrodes which have a large enough diameter to carry the required current, e.g., 1 ampere.
Moreover, electrophysiology catheters typically have nearly flush platinum or platinum/iridium electrodes which are less thrombogenic and traumatic. Where there is a congested arrangement of elements within the tip cross section so that these flush rings cannot be buried within the tip, the ring electrodes are typically thin, usually about 0.003 inch. These rings must be galvanically stable, biocompatible and extremely strong so that they do not break and fall off the tip and be left in the patient. Thin platinum iridium rings are typically chosen. However, thin platinum iridium rings have very poor circumferential heat transfer. Hence, they are not compatible with a single thermocouple design for predicting the temperature at all points along the ring except for the area directly over the thermocouple.
Accordingly, there is a need for a catheter construction which allows for multiple site ablation with accurate temperature monitoring means while still maintaining a small overall catheter diameter.